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I have an equation such as:

a*z^2 + b*(z - c)^2 == d

where z is a complex variable and a, b, c, d are all complex parameters. I want to put x+I*y instead of z and decompose this equation into two pure real and imaginary part for further analysis, but I don't know how to do it by Mathematica! Can anybody help me?

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Try: 

ComplexExpand[a*z^2 + b*(z - c)^2, {_}]

Or to get a somewhat shorter result:

ComplexExpand[a*z^2 + b*(z - c)^2, {_}] /. a_ + Complex[0, 1] b_ :> Simplify[a] + I Simplify[b]
(*
 -Im[c]^2 Re[b] - Im[z]^2 (Re[a] + Re[b]) + Re[b] Re[c]^2 - 
  2 Re[b] Re[c] Re[z] + Re[a] Re[z]^2 + Re[b] Re[z]^2 + 
  2 Im[b] Im[c] (-Re[c] + Re[z]) + 
  2 Im[z] (Im[c] Re[b] + Im[b] (Re[c] - Re[z]) - Im[a] Re[z]) + 
 I (Im[b] (-Im[c]^2 + 2 Im[c] Im[z] - Im[z]^2 + (Re[c] - Re[z])^2) + 
    Im[a] (-Im[z]^2 + Re[z]^2) + 2 (Im[c] Re[b] (Re[c] - Re[z]) + 
    Im[z] (Re[a] Re[z] + Re[b] (-Re[c] + Re[z]))))
*)
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